1. Field of the Invention
The present invention relates to an optical device that includes an optical element such as a wavelength converting element that varies the emitted wavelength of laser light, and particularly relates to an optical device that includes a means of correcting temperature characteristics of the wavelength converting element.
2. Description of the Related Art
Short-wavelength (e.g., blue and green) laser optical sources are under extensive development in the fields of, for example, laser projectors and high density optical storage. These short-wavelength laser optical sources output blue or green laser light by converting into a harmonic, the infrared light of a fundamental wave output by a semiconductor laser. Here, the wavelength converting elements used are made of lithium niobate (LiNbO3) as a primary component, nevertheless, the conversion efficiency of the harmonic is temperature dependent and has a characteristic of varying greatly according to temperature variations.
FIG. 12A is a graph depicting an example of variation in harmonic output versus environmental temperature of a wavelength converting element. As can be understood from the graph, when the environmental temperature is in a low region, the wavelength converting element output drops and even when the environmental temperature is in a high region, the output drops. In this manner, since the harmonic output by the wavelength converting element varies greatly with respect to temperature, conversion of good efficiency is realized and to obtain laser light having a stable harmonic, a temperature adjusting means for correcting the temperature characteristic of the wavelength converting element is essential. To adjust the wavelength converting element to a given temperature, a laser optical source having a heater disposed at the wavelength converting element is known (see, for example, Japanese Patent Application Laid-Open Publication No. H6-338650 (page 5, FIG. 5)).
FIG. 12B depicts an example of the short-wavelength laser optical source disclosed in Japanese Patent Application Laid-Open Publication No. H6-338650. In FIG. 12B, reference numeral 101 is a silicon substrate; reference numeral 110 is a 0.8 μm-band semiconductor laser; and reference numeral 120 is a wavelength converting element. A fundamental wave 112 is output from an active layer 111 of the semiconductor laser 110, input to a waveguide 121 of the wavelength converting element 120, and a harmonic of blue laser light 130 is output. At a portion of a surface of the silicon substrate 101 (a surface contacting the wavelength converting element 120), a groove 102 is formed by etching.
Further, at a lower portion of the wavelength converting element 120, i.e., near the waveguide 121, a thin film heater 122 is formed of a Ti film. By energizing the thin film heater 122, the temperature of the wavelength converting element 120 can be maintained at a given temperature. Furthermore, the thin film heater 122 does not contact the silicon substrate 101 because of the groove 102, whereby a configuration that makes it difficult for the heat of the thin film heater 122 to be transferred to the silicon substrate 101 is formed. Consequently, the temperature of the wavelength converting element 120 is maintained at a given temperature and stable output of the laser light is shown.
A configuration is further known where a heater for adjusting the temperature of the wavelength converting element is not formed at the wavelength converting element, but rather on the substrate; and the substrate and the semiconductor laser are bonded by a soldering pattern or an adhesive having a high-thermal conductivity, whereby the heat generated by the heater is transferred to the semiconductor laser (see, for example, Japanese Patent Application Laid-Open Publication No. 2003-17793 (page 3, FIG. 1)). The laser optical source of this patent document has a configuration in which a thin film heater is formed on an insulation film. On the thin film heater, a soldering pattern is formed mounting a semiconductor laser chip that includes a variable wavelength region. Consequently, by energizing the thin film heater on the substrate, heat from the heater is transferred through the soldering pattern to the semiconductor laser chip, enabling adjustment of the temperature of the variable wavelength region.
Nevertheless, in the configuration recited in Japanese Patent Application Laid-Open Publication No. H6-338650, since the thin film heater is formed on the wavelength converting element, a problem arises in that a process of forming the thin film heater is added to the fabrication process of the wavelength converting element, increasing the number of fabrication processes as well as man-hours. Furthermore, wiring and electrodes for energizing the thin film heater on the wavelength converting element must be disposed at both the wavelength converting element and the silicon substrate, whereby as a laser optical source, the structure becomes complicated, making it easy for problems such as poor electrode contact to arise.
In the configuration recited in Japanese Patent Application Laid-Open Publication No. 2003-17793, a problem arises in that although the substrate and the semiconductor laser chip are bonded by soldering, bonding the semiconductor laser chip by a uniform soldering coating to efficiently transfer the heat generated by the heater to the semiconductor laser chip is difficult. Furthermore, although obviously, soldering requires an application of heat to melt the soldering material and with the application of heat, there is a risk of damage to the substrate and/or the semiconductor laser chip. Moreover, errant positioning of the substrate and the semiconductor laser chip occurs easily with soldering.